In general, an impregnated cathode for a CRT is manufactured by pressing tungsten powder into a pellet having a certain porosity, sintering the pellet, impregnating electron emitting material into the sintered pellet, and securing the pellet to a pellet sleeve and a heater sleeve, generally using a laser or an electric-resistance welding method or a brazing method. Therefore, since the pellet is secured to the heater sleeve by the welding, the impregnated electron emitting material is deteriorated due to an effect of the welding heat and hot electron emitting decreases, or hot electron is not produced from the cathode, thus deteriorating the whole characteristics of the CRT.
FIGS. 1 to 5 illustrate the improved prior art cathode structures to solve or alleviate the above problems or the like.
Referring to FIG. 1, a pellet cup 12, which is secured to a heater sleeve 13, has plural projection parts on its base part in an impregnated cathode structure, which is disclosed by Japanese patent laid-open publication No. 61227342 A. Thus, a substantial contact area with a cathode pellet 11 can be kept large even when a pellet cup 12 generates thermal deformation. Thereby, the fitting condition between the cathode pellet 11 and the pellet cup 12 can be stabilized to realize an impregnated cathode having stable electron emitting property at its high operating temperature and thereby improve the temperature property of a cathode.
In FIG. 2, a pellet 21 of an impregnated cathode structure, which is disclosed by Japanese patent laid-open publication No. 55143743 A, is obtained by cutting a porous tungsten rod which is sintered after compressing tungsten powder. A pellet assembly is formed by securing tungsten wire mesh 22 tightly through Mo-Ru brazing material 23 mixed with an organic binder to the pellet 21. Then the outside portion is removed such that the surface portion of the mesh 22 is exposed. Thereafter it is heated under reductive ambient to perform the brazing work. Then said substrate is cut to predetermined shape to produce a cathode member which is welded through resistor welding with a heater sleeve, which is not illustrated herein. This structure can achieve good soldering and stable characteristics.
FIG. 3a illustrates the prior art impregnated cathode structure for a CRT, which is disclosed on Japanese patent laid-open publication No. 03155020 and wherein an intermetallic compound bond layer 33 with a main ingredient of Al is formed between a pellet 31 and the bottom surface of a cup 32 to firmly bond them together, then the cup 32 is secured to the upper surface of the a heater sleeve 34. This provides an impregnated cathode structure showing stabilized electron emitting characteristic even after a long period of use. In FIG. 3b, the intermetallic compound bond layer 33 is formed by placing an aluminum foil 33' between the pellet 31 and the bottom surface of the cup 32 followed by heating in vacuum, thus having an advantage of a low manufacturing cost.
Referring to FIG. 4 disclosed on Japanese patent laid-open publication No. 60165021, many holes 42 are formed on a nickel substrate 43 and are impregnated with the alkaline earth carbonate. Furthermore, the alkaline earth carbonate may be spread on the electron emission layer 41 to such a thickness that coating resistance generates no trouble. In this way, even if the cathode is heated to high temperature during the sealing procedure of the cathode-ray tube in atmosphere, the nickel substrate 43 does not take much oxygen and the cathode-ray tube after completion stably bears high current density operation, thereby reproducing the images with high brightness and high precision.
In FIG. 5, which illustrates the prior art impregnated cathode structure disclosed by Japanese patent laid-open publication No. 60047331, brazing material 54 is applied to only the external section of a hollow cylindrical body 55 that is integratedly comprised with a cathode substrate 51. Since such structure does not require the sealing hole treatment near the central part in which the thickness of the cathode substrate 51 is smallest, the insufficient impregnation of electron emission material can be offset due to the intrusion of brazing material and a cathode with more homogeneous electron emission characteristics can be obtained. In addition, since a heater coil 53 is connected to the inner part of a sleeve 52 and the hollow cylindrical body 55, thermal conduction efficiency is improved.
However, the above conventional impregnated cathode structures still employ a welding method such as a laser welding method or a brazing method in securing the pellet to a pellet sleeve and a heater sleeve. Therefore, the above conventional impregnated cathode structures have not completely overcome the problem yet that the impregnated electron emitting material is deteriorated due to an effect of the welding heat, which thus causes deterioration of the whole characteristics of the CRT.